1. Field of the Invention
The present invention relates generally to the field of respiratory assist devices for respiratory care of a patient and, more particularly, to a respiratory system that automatically and variably on demand regulates the supply of a pressurized gas source to provide the appropriate pressure assist to the patient.
2. Background
Mechanical ventilatory support is widely accepted as an effective form of therapy and means for treating patients with respiratory failure. Ventilation is the process of delivering oxygen to and washing carbon dioxide from the alveoli in the lungs. When receiving ventilatory support, the patient becomes part of a complex interactive system which is expected to provide adequate ventilation and promote gas exchange to aid in the stabilization and recovery of the patient.
In those instances which a patient requires mechanical ventilation due to respiratory failure, a wide variety of mechanical ventilators are available. Most modern ventilators, i.e., respiratory systems, allow the clinician to select and use several modes of inhalation either individually or in combination via the ventilator setting controls that are common to the ventilators. These modes can be defined in three broad categories: spontaneous, assisted, or controlled. During spontaneous ventilation without other modes of ventilation, the patient breathes at his own pace, but other interventions may affect other parameters of ventilation including the tidal volume and the baseline pressure, above ambient, within the system. In assisted ventilation, the patient initiates the inhalation by lowering the baseline pressure by varying degrees, and then the ventilator xe2x80x9cassistsxe2x80x9d the patient by completing the breath by the application of positive pressure. During controlled ventilation, the patient is unable to breathe spontaneously or initiate a breath, and is therefore dependent on the ventilator for every breath. During spontaneous or assisted ventilation, the patient is required to xe2x80x9cworkxe2x80x9d (to varying degrees) by using the respiratory muscles in order to breath.
The work of breathing (the work to initiate and sustain a breath) performed by a patient to inhale while intubated and attached to the ventilator may be divided into two major components: physiologic work of breathing (the work of breathing of the patient) and breathing apparatus imposed resistive work of breathing. The breathing apparatus is properly defined as the endotracheal tube, the breathing circuit including an inhalation conduit, an exhalation conduit and a xe2x80x9cYxe2x80x9d piece, the gas regulator, such as a ventilator, and may include a humidifier. The work required to spontaneously inhale through the breathing apparatus is the imposed resistive work of breathing (WOBi). The work of breathing can be measured and quantified in Joules/L of ventilation. In the past, techniques have been devised to supply ventilatory therapy to patients for the purpose of improving patient""s efforts to breath by decreasing the work of breathing to sustain the breath. It is desirable to reduce the effort expended by the patient since a high work of breathing load can cause further damage to a weakened patient or be beyond the capacity or capability of small or disabled patients.
In conventional respiratory systems, pressure required to inflate the lungs of a patient connected to a life-support mechanical ventilator is typically measured within the breathing circuit from one of three conventional sites: the inspiratory conduit; the expiratory conduit; or the xe2x80x9cYxe2x80x9d piece which is connected to both the inspiratory conduit and the expiratory conduit. Conventionally, at the onset of spontaneous inhalation in the pressure support ventilation (PSV) mode, for example, a pressure change is detected in the breathing circuit, the ventilator is triggered xe2x80x9con,xe2x80x9d and lung inflation is assisted by positive pressure supplied by the ventilator. Pressure is then monitored/controlled from the pressure measuring site to ensure the preselected pressure is achieved, and with some ventilators, the rate of pressure rise over time is controlled based on preselected settings. At a specific pressure, for ventilatory support modes relying on pressure for cycling to the xe2x80x9coffxe2x80x9d phase, inhalation is terminated.
Inaccurate pressure measurements on the airways and lungs result from using the aforementioned conventional pressure measuring sites. During spontaneous breathing with continuous positive airway pressure (CPAP), for example, significant underestimations of pressure result compared with measuring tracheal pressure at the distal end of an endotracheal tube (PT). This is especially true when using narrow internal diameter endotracheal tubes and when peak spontaneous inspiratory flow demands are high. The narrower the tube and the greater the flow rate demand, the greater the discrepancy or inaccuracy between pressure measured at the xe2x80x9cYxe2x80x9d piece and PT.
Due to inherently resistive components within the breathing circuit, in which the endotracheal tube is regarded as being the most significant resistor, the further from the trachea pressure is measured, the smaller the deviations in pressure that occur with CPAP. Small deviations during spontaneous inhalation may lead the clinician to erroneously conclude that the flow rate provided on demand by the ventilator is sufficient and the imposed resistive work of the breathing apparatus is minimal when, in fact, large deviations in pressure at the distal end of the endotracheal tube and a highly resistive workload imposed by the apparatus may be present.
Overestimations in tracheal airway pressure may result during mechanical inflation from the use of the conventional pressure measuring sites. Peak inflation pressure (PIP) generated during mechanical inflation varies directly with total resistance (imposed endotracheal tube resistance and physiologic airways resistance) and inversely with respiratory system compliance (Crs). PIP measured at the xe2x80x9cYxe2x80x9d piece reflects the series resistance of the endotracheal tube and physiologic airways as well as Crs. Peak pressure measured in the trachea reflects physiologic resistance and Cr, only. Therefore, PIP measured at the xe2x80x9cYxe2x80x9d piece may be greater than in the trachea. The narrower the internal diameter of the endotracheal tube and the greater the mechanical inspiratory flow rate, the more PIP measured at the xe2x80x9cYxe2x80x9d piece overestimates pressure at proximate the distal end of the endotracheal tube. This becomes especially critical when dealing with intubated pediatric patients.
The conventional pressure measuring sites also compromise the responsiveness of the conventional respiratory systems to patient inspiratory efforts. This produces delays in triggering the supply of inspiratory assist xe2x80x9conxe2x80x9d and cycling xe2x80x9coff,xe2x80x9d predisposing the patient to patient-respiratory system dysynychrony, and can dramatically increase the effort or work to inhale. Significantly less imposed work results from pressure triggering and controlling the respiratory system xe2x80x9conxe2x80x9d at the distal end of the endotracheal tube compared with the conventional method of pressure triggering and controlling or by using Flow-By(trademark). Flow-By(trademark) is a method introduced by the Mallingckrodt Nellcor Puritan-Bennett Company that uses flow sensitivity to trigger the ventilator xe2x80x9conxe2x80x9d instead of pressure sensitivity. Whether pressure triggering from inside the ventilator or using Flow-By(trademark), an initial pressure drop across the endotracheal tube must be generated by the patient to initiate flow. This effort results in significant increases in imposed resistive work of breathing. Conventional respiratory systems introduce other more complex factors contributing to WOBi: the work to trigger the ventilator xe2x80x9conxe2x80x9d (i.e., to initiate flow), the relative flow or pressure target used during the post trigger phase, and breath termination or cycling xe2x80x9coffxe2x80x9d criteria.
Thus, the design of conventional respiratory systems results in respiratory support systems that are predisposed to increase WOBi or have limited ability to decrease the WOBi. The conventional designs also fail to provide any reliable degree of automatic responsiveness. To compensate, modern conventional respiratory systems contain complicated control algorithms so that the ventilators can xe2x80x9capproximatexe2x80x9d what is actually occurring within the patient""s lungs on a breath-by-breath basis. In effect, the computer controlled prior art respiratory systems are limited to the precise, and unyielding, nature of the mathematical algorithms which attempted to mimic cause and effect in the ventilator support provided to the patient. Ventilatory support should be tailored to each patient""s existing pathophysiology and should both provide automatic and variable levels of pressure assist in response to patient inspiratory demand and should minimize WOBi. Such a respiratory system is unavailable in current ventilators.
In accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to a respiratory system that supplies a pressurized breathing gas from a pressurized gas source to a patient via a breathing circuit in fluid communication with the lungs of a patient. The breathing circuit includes an endotracheal tube, an inhalation conduit in fluid communication with the proximal end of the endotracheal tube, and an exhalation conduit in fluid communication with the proximal end of the endotracheal tube. The respiratory system includes a demand valve in selective fluid communication with the pressurized gas source and the inhalation conduit and a tracheal pressure conduit in fluid communication with the distal end of the endotracheal tube and a reference camber.
The demand valve is movable between a first position, in which the demand valve is opened so that the inhalation conduit is placed in fluid communication with the pressurized gas source, and a second position, in which the demand valve is closed so that the inhalation conduit is not in fluid communication with the pressurized gas source. The respiratory system further includes a means for proportionally opening the demand valve in response to pressure changes within the reference chamber so that WOBi is automatically nullified by providing levels of inspiratory pressure assist that are automatic and proportional on demand of the patient.